This invention relates to apparatus for measuring surface roughness and, more particularly, it concerns improvements in apparatus employing the "total integrated scatter" method of measuring surface roughness.
Manufacturers of equipment requiring precision surfaces such as calendering rolls and optical equipment including lenses, mirrors and the like, are increasingly called upon to characterized the predicted performance capabilities of their products in terms of specified microroughness. While a wide variety of techniques are known for measuring and quantifying microroughness in the range of from 100 .ANG. RMS to less than 10 .ANG. RMS, presently available equipment capable of reliable applying the known roughness measuring technology suffers from such disadvantages as being excessively expensive, difficult to use, and/or time consuming in operation.
Of the many available methods for providing an indication of microroughness, the Total Integrated Scatter of T.I.S. method of surface evaluation has substantial potential for incorporation in equipment which does not suffer from the aforementioned disadvantages. In the T.I.S. method, the surface of a test sample is placed at one off-axis focus of an interiorly polished hemisphere, such as a Coblentz Sphere, and illuminated at normal incidence with an HE-NE laser entering an aperture at or near the apex of the hemisphere. As a result of the normal or near normal incidence of the laser illumination, specularly reflected laser light exits through the same aperture whereas all laser light scattered by roughness of the same surface (excepting only that light which exits through the apex aperture) is focused at the conjugate focus of the hemisphere. Surface roughness of the sample can be quantifified by the different intensities of the specularly reflected light and the focused scattered light in accordance with the equation: EQU h.sub.RMS (.ANG.)=(6328.ANG./4.pi.)((V.sub.d /V.sub.RS)(r.sub.RS /r.sub.S)).sup.1/2
where V.sub.d is a signal corresponding to the intensity of scattered light, V.sub.RS is a reference scatter signal, r.sub.RS is the reference specular signal, and r.sub.S is a signal corresponding to the intensity of specular light reflected from the sample.
While the T.I.S. method provides good relative measurements of surface microroughness in the context of sensitivity, providing a single number for specification and avoiding physical contact with a sample surface, several problems are presented to the provision of a low-cost, effective apparatus for its practice. For example, because a hemisphere has only one true focal point (i.e., its center), it is not possible to locate both a sample surface and a detector simultaneously at that true focal point. As a result, one or both of the sample and detector or sensor must be located off the true focal point, in turn resulting in laser light scattered from the sample being not focused at a point by the interior hemispheric reflective surface, but rather at an area which may be an order of magnitude larger than the sample illuminating laser beam. The size of the area of scatter light focus is further enlarged by imperfections in the internal hemispheric reflective surface. Accordingly, signficant increments of costs have been attributed in prior apparatus to scatter light detection devices and to the quality of the hemispheric reflective surface.
Another challenge to apparatus for practicing the T.I.S. method of surface roughness measurement is the requirement that the sensors for measuring specular reflection and scatter focus be matched for the full range of surface roughness to be measured. This challenge has been met in the past by using the same sensor to measure both values of light intensity consecutively. Because the single sensor must be manipulated for each measurement, the measurement procedure is made time consuming and tedious to a point where accuracy may be compromised. In addition, the physical presence of the sensor structure within the hemisphere can block some of the scatter light passing from the sample to the interior hemispheric reflective surface. Since the direction of scatter light eminating from the sample may vary without change in the total amount of scatter, any interference with scatter light reaching the interior hemispheric reflector will result in a potential inaccuracy of the roughness measurement.
In light of the foregoing, it will be apparent that while the methodology for accurate measurement of surface microroughness is presently available, there is a need for improvement in apparatus for practicing the art.